Thread forming fasteners for ultrasonic load measurement and control

ABSTRACT

An ultrasonic load measurement transducer is mated with a thread-forming fastener to provide a load indicating thread-forming fastener that can be used for the precise and reliable assembly of critical bolted joints, such as those in the automobile and aerospace industries, among others. Steps can then be taken to accurately measure and control the load in the thread-forming fastener during tightening, and to inspect the load in the thread-forming fastener after assembly. A similar result can be achieved for a thread-locking fastener by mating an ultrasonic transducer with the thread-locking fastener assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/287,761, filed Oct. 14, 2008, which has since issued as U.S. Pat. No.7,644,627, on Jan. 12, 2010. U.S. patent application Ser. No. 12/287,761is, in turn, a divisional of U.S. patent application No. 11/344,028,filed Jan. 31, 2006, which has since issued as U.S. Pat. No. 7,467,556,on Dec. 23, 2008. U.S. patent application Ser. No. 11/344,028 is in turna continuation-in-part of U.S. patent application Ser. No. 10/528,515,which has an assigned filing date of Oct. 26, 2005, which was filed asthe National Stage of International Application No. PCT/US2003/029302,filed Sep. 19, 2003, and which claims the benefit of U.S. ProvisionalApplication No. 60/412,125, filed Sep. 19, 2002. U.S. patent applicationNo. 10/528,515 is also a continuation-in-part of U.S. patent applicationNo. 10/470,372, which has an assigned filing date of Jul. 25, 2003,which has since issued as U.S. Pat. No. 6,990,866, on Jan. 31, 2006,which was filed as the National Stage of International Application No.PCT/US02/03920, filed Jan. 28, 2002, and which claims the benefit ofU.S. Provisional Application No. 60/264,877, filed Jan. 29, 2001.

BACKGROUND OF THE INVENTION

This invention relates to load indicating fasteners that are“thread-forming” (also referred to as “thread-rolling” or “self-tapping”fasteners), methods for making load indicating thread-forming fasteners,and methods for measuring the load in thread-forming fasteners.

Thread-forming fasteners are well known in many industries, such as inhigh-volume automotive assembly. Examples of such fasteners aredescribed in U.S. Pat. No. 5,242,253 (Fulmer), issued Sep. 7, 1993, forexample. Such fasteners are also marketed commercially, for example, byReminc, Research Engineering and Manufacturing Inc., Middletown, RI,USA, under the trademark “Taptite” and “Taptite 2000”, and a descriptionof such fasteners can be found in their product literature, entitled“Taptite 2000 Thread Rolling Fasteners”.

The major advantage of thread-forming fasteners is that they can beinstalled directly into a drilled hole, eliminating the cost of tappingthe hole. Additionally, the thread formed by a thread-forming fastenerhas very tight tolerance since it is formed by the fastener itself andtherefore forms a better nut.

Although thread-forming fasteners have been used in numerousapplications in the automotive and aerospace industries to reduce cost,such fasteners are generally restricted to non-critical or less-criticalapplications. The difficulty in controlling the tightening processprevents their use in critical applications.

The primary reason for this is that the thread-forming process requirestorque, in addition to the tightening torque, and this thread-formingtorque varies significantly with hole tolerance, material, frictionconditions, etc. As a result, the precise tightening of a thread-formingfastener to a specified torque into a blind hole, where the thread isstill being formed as the bolt is being tightened, will result in a 3sigma load scatter of typically +/−50%, which is unacceptable incritical applications.

SUMMARY OF THE INVENTION

For some time, ultrasonics has been used to accurately measure the loadin bolts. Initially, removable ultrasonic devices were the most commonlyused. More recently, low-cost permanent ultrasonic transducers, whichcan be permanently attached to one end of the fastener, have come to beused. Permanent fasteners of this type are described, for example, inU.S. Pat. No. 4,846,001 (Kibblewhite), issued Jul. 11, 1989, U.S. Pat.No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992, U.S. ProvisionalPatent Application No. 60/264,877 (Kibblewhite), filed Jan. 29, 2001,and International Application No. PCT/US02/03920 (Kibblewhite), filedMay 17, 2002, the subject matter of which is incorporated by referenceherein.

In accordance with the present invention, it has been determined thatsuch ultrasonics can be mated with an otherwise conventionalthread-forming fastener to provide a load indicating thread-formingfastener that can be used for precise and reliable assembly of criticalbolted joints, such as those in automobile engines (e.g., cylinderheads, connecting rods, main bearings, etc.), drive trains, steering,brakes, suspensions, and a variety of other applications, includingaerospace applications.

Steps can then be taken, using equipment and methods that are otherwiseknown and conventional, to accurately measure and control the load inthe thread-forming fastener during tightening, and to inspect the loadin the thread-forming fastener after assembly.

For further detail regarding preferred embodiments for implementing theimprovements of the present invention, reference is made to thedescription which is provided below, together with the followingillustrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a typical load indicating thread-formingfastener which is produced in accordance with the present invention.

FIGS. 2 and 3 are graphs showing typical load and torque characteristicsplotted against the angle of rotation of the load indicatingthread-forming fastener of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a typical embodiment of a load indicating thread-formingfastener which is produced in accordance with the present invention. Inthis illustrative example, the load indicating thread-forming fastenerhas been implemented in conjunction with an otherwise conventional“Taptite” fastener, which is commercially available from Reminc,Research Engineering and Manufacturing Inc., Middletown, R.I., USA. Itis to be understood, however, that this embodiment is shown only forpurposes of illustration, and that the load indicating thread-formingfastener of the present invention can also be implemented using any of avariety of known and available load indicating devices, coupled orcombined with any of a variety of known and available thread-formingfasteners.

In the illustrative embodiment of FIG. 1, the load indicatingthread-forming fastener 10 generally includes a fastener 12 (e.g., theabove-mentioned “Taptite” fastener) and a permanent piezoelectricpolymer film transducer 14 (e.g., of the type disclosed in theabove-mentioned U.S. Pat. No. 4,846,001, issued to Kibblewhite) attachedto one end. The fastener 12 includes a head 16, which can be suitablyengaged by a tool (not shown) for applying torque to the fastener 12,and a shank having a thread-forming body portion 18.

A suitable identifying element is applied to the thread-forming fastenerwhich can be read and used to determine ultrasonic measurementparameters specific to the thread-forming fastener in order to providemore precise and more reliable load measurements by compensating fordifferences resulting from manufacturing variations in individualthread-forming fasteners.

For example, as disclosed in U.S. Provisional Patent Application No.60/264,877 (Kibblewhite) and International Application No.PCT/US02/03920 (Kibblewhite), the transducer 14 can further include apermanent mark such as a two-dimensional high-density bar code (notshown) or some other encodable medium, applied to the top electrode 20of the transducer 14 for purposes of facilitating subsequent steps takento obtain an indication of tensile load, stress, elongation or othercharacteristic of the fastener 12 during a tightening operation, or atvarious other times during the service life of the fastener 12, as willbe discussed more fully below.

As an alternative, the permanent mark can be applied directly to thethread-forming fastener, and the ultrasonic transducer can then beapplied on top of the mark in such a way that the mark can be detectedthrough the transducer. As an example, the bar code can be marked on anend surface of the fastener and the ultrasonic transducer can then beprovided on the surface with the bar code in such a manner that the barcode can be read through the transducer. In one such embodiment, thetransducer layers are translucent or transparent, allowing the bar codeto be read through the piezoelectric and conductive layers of thetransducer. In another embodiment, the bar code is marked using anindentation technique, such as dot peening, so that the indentations aredetectable, and the bar code is made readable, after application of thetransducer.

As a further alternative, a non-volatile memory device can be applied tothe thread-forming fastener for purposes of storing desired information.Such memory devices can be powered, written to and read from seriallythrough a single input/output connection and an AC coupled returnthrough the capacitance of the ultrasonic transducer. Such devices arecapable of storing data such as unique identification, ultrasonicmeasurement parameters, tightening and inspection data for use in amanner similar to that of the above-described use of a permanent markfor the storage of information.

In one such embodiment, the previously described top electrode 20 isreplaced with the non-volatile memory device, and portions of the topexposed surface of the memory device are made conductive by providingthe surface with an electrical contact. This top conductive surface isthen electrically connected to a conductive layer on the bottom of thememory device, adjacent to the active piezoelectric polymer filmtransducer 14, to provide a suitable electrode for the ultrasonictransducer. The top conductive surface is also electrically connected tothe non-volatile memory device for purposes of writing information toand reading information from the memory device.

In another embodiment, the foregoing non-volatile memory device can be aradio frequency identification (RFID) chip or tag coupled with thetransducer 14 for purposes of storing desired information. This can beaccomplished with known RFID devices, such as the MetalSentinel (13.56MHz) device available from Interactive Mobile Systems, Inc., PortTownsend, Wa., USA, which are capable of storing data such as uniqueidentification, ultrasonic measurement parameters, and tightening andinspection data.

In such an embodiment, the previously described top electrode 20 isreplaced with the RFID device, and portions of the top exposed surfaceof the RFID device are made conductive by providing the exposed surfacewith an electrical contact. This top conductive surface is thenelectrically connected to a conductive layer on the bottom of the RFIDdevice, adjacent to the active, piezoelectric polymer film transducer14, to provide a suitable electrode for the transducer 14. Thepiezoelectric polymer film transducer 14 is an electrical insulator andfurther functions as an isolator for the antenna associated with theRFID device for purposes of RF transmission.

The size, shape and location of the conductive portions of the topexposed surface of the RFID device can vary to suit the particular RFIDdevice which is used. For example, the conductive portions of the topexposed surface can be placed along the periphery of the RFID device,leaving the central portions of the top exposed surface open toaccommodate the antenna normally associated with the RFID device. Theconductive portions of the top exposed surface should preferably coveras much of the top surface of the RFID device as is possible, whileleaving sufficient open space to accommodate the function of theantenna. The conductive layer on the bottom of the RFID devicepreferably covers the entire bottom surface, to maximize contact withthe transducer 14.

Various different couplings are used with RFID devices, includingelectromagnetic, capacitive and inductive couplings, with differentcoupling antennas. The antenna can be provided adjacent tonon-conductive portions of the top exposed surface. Alternatively, theconductive portions of the top and bottom surfaces of the RFID devicecan be constructed in such a way as to function as the antenna for thetransponder associated with the RFID device which is used. It willfurther be appreciated that non-contact inductive or capacitivecouplings used for RFID transponder communication in the above describedembodiments can also be used to couple the excitation signal to theultrasonic transducer. Additionally, the RF communication frequency canbe selected to correspond to a preferred ultrasonic transducerexcitation frequency. This then eliminates the need for an electricallyconductive top surface for electrical contact with the transducer forload measurement, allowing both the reading of information stored in theRFID device and the measurement of load to be performed even when thetransducer is covered with paint or other protective coating.

As an example, the transducer 14 can be implemented using a thinpiezoelectric polymer sensor (e.g., a 9 micron thick, polyvinylidenefluoride copolymer film, of the type manufactured by MeasurementSpecialties Inc., Valley Forge, Pa., USA) permanently, mechanically andacoustically attached to an end surface 22 of the fastener 12. The topelectrode 20 of the transducer 14 can be implemented as a thin metallicfoil (e.g., an approximately 50 micron thick, type 316, full-hard, dullor matte finished stainless steel) which has been treated to provide ablack oxide finish, which is then preferably provided with a black oxidetreatment to provide an extremely thin, durable, corrosion resistant andelectrically conductive, black coating. A high-resolution bar code canbe marked on the resulting surface by removing selected areas of thecoating (e.g., by conventional laser ablation techniques), or by someother process, to provide a high contrast mark easily read withconventional, commercially available optical readers. As an alternative,a non-volatile memory device, such as an RFID device, can be applied tothe transducer 14 to provide data storage which can similarly be readwith conventional, commercially available readers.

It is again to be understood that such implementations are describedonly for purposes of illustration, and that any of a variety oftransducer configurations can be used to implement the transducer 14applied to the fastener 12, as desired. For example, the ultrasonictransducer 14 can be implemented as an oriented piezoelectric thin film,vapor deposited directly on the head of the fastener 12, as is describedin U.S. Pat. No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992. As afurther alternative, the ultrasonic transducer 14 can be implemented asa piezoelectric polymer film, chemically grafted on the head of thefastener 12, as is described in U.S. Provisional Patent Application No.60/264,877 (Kibblewhite), filed Jan. 29, 2001, and InternationalApplication No. PCT/US02/03920 (Kibblewhite), filed May 17, 2002. Itwill be readily understood that other alternative implementations arealso possible.

In the embodiment illustrated in FIG. 1, the ultrasonic transducer 14 ispermanently attached to the head 16 of the fastener 12, as described inthe above-referenced patents issued to Kibblewhite. An essentially flat,or spherically radiused surface 24 is provided on at least a portion ofthe threaded end of the fastener to provide an acoustically reflectivesurface to reflect the ultrasonic wave transmitted by the transducerback to the transducer. Load is then measured using standard, pulse-echoultrasonic techniques, which are themselves known in the art anddescribed, for example, in the above-referenced patents issued toKibblewhite. Load control accuracies of +/−3% have been achieved whentightening thread-forming fasteners into blind holes during both thefirst and subsequent tightenings.

In an alternative embodiment, an essentially flat surface is provided onthe head 16 of the thread-forming fastener 12 and a removable ultrasonictransducer is temporarily attached to the fastener for the purpose ofmaking load measurements. The threaded end of the fastener 12 isidentical to the previous embodiment with the permanent ultrasonictransducer.

In practice, heat is generated as a result of the thread-forming workthat takes place during the thread-forming run-down stage of theinstallation of a thread-forming fastener. This results in a slightincrease in temperature in both the fastener (the bolt) and theresulting joint. This increase in temperature can cause errors in theultrasonic load measurements to be taken because of thermal expansioneffects. For this reason, when using ultrasonics for inspecting the loadin a fastener, it is usual to measure the temperature of the fastener orthe joint in order to compensate for the effects of thermal expansion.

However, in conjunction with a thread-forming fastener, the averagetemperature increase due to the heat generated during thread-formationcan not be measured directly during the installation process and issubject to variations in material, friction, and heat conductionproperties of the joint components. Without compensation, this thermaleffect can result in inaccuracies of load measurement on the order of 5%to 20%, depending on the bolt, the joint and the assembly process beingused.

FIGS. 2 and 3 show typical load and torque characteristics plottedagainst the angle of rotation of a typical bolt. FIG. 2 shows thetightening curves for a typical through-hole application, in which thetorque reduces after the thread is formed through the entire hole. FIG.3 shows the tightening curves for a typical blind hole application, inwhich the thread is still being formed as the bolt is tightened.

Further in accordance with the present invention, more accurate loadmeasurements in the thread-forming load indicating fasteners areprovided by eliminating the effects of fastener heating resulting fromthe thread-forming process. This is achieved by measuring the load (oracoustic time-of-flight) value immediately prior to the load-inducingstage of the assembly process, and by using this measured value as thezero-load reading.

The load-inducing stage of the assembly process can be detected by anyone of a variety of methods. For example, an increase in load above apredetermined threshold, a change in the increase in load with time,angle of rotation of the fastener or torque, an increase in torque abovea predetermined threshold, or a change in the increase in torque withtime, angle or load can be detected. Irrespective of the method used todetect the load-inducing stage of the assembly process, a new zero-loadbase measurement is taken as a value just prior to the load-inducingassembly stage by selecting or calculating a load measurement prior tothe load-inducing stage. This can be achieved by selecting a loadmeasurement corresponding to a fixed time or angle prior to thedetection of the commencement of the load-inducing stage, for example.Alternatively, for through-hole applications, the end of thethread-forming phase can be detected by a reduction in torque. It isagain to be understood that such methods are only illustrative, and thatthere are numerous other methods for determining the new zero-load basemeasurement prior to tightening, from load, time, torque and angle ofrotation measurements recorded during assembly operations with hand andpowered assembly tools.

The thermal effect of thread forming causes an apparent positive loadvalue at zero load just prior to tightening. An alternative to zeroingthe load (or time-of-flight measurement) is to add this load offset,measured prior to the load-inducing stage of the assembly process, tothe target load (or target time-of-flight). The result is the same sincethe increase in measured load is the same.

Yet another alternative is to experimentally determine an average valueof load error due to the thread forming and adjust the zero-loadmeasurement or target tightening parameter to compensate for this effectusing one of the above-described methods. This approach, however, doesnot compensate for variations with individual fasteners or jointcomponents and is therefore presently considered less desirable.

The result is that, for the first time, ultrasonic load measurementtechnology can be used with thread-forming fasteners. Errors in loadmeasurement resulting from the thermal effects of thread-forming can becompensated. This then results in accurate load measurement andtightening control of the thread-forming fasteners.

The above-described method of eliminating the effects of fastenerheating resulting from the thread-forming process can also be used withother fastener assembly processes that generate heat prior to theload-inducing tightening stage. Thread-locking bolts and nuts, forexample, are manufactured with a prevailing “locking” torque to preventthe fastener from loosening during service. Most often, the thread ofeither the bolt or nut has an irregular profile causing the threads toelastically deform slightly upon mating. Alternatively, the bolt or nuthas an insert or patch of a soft material to provide the prevailingtorque or resistance to loosening. The prevailing torque provided bythese thread-locking features produces heating of the fastener duringrundown in the same manner as the tapping torque does with athread-forming fastener. Consequently, the above-described method forcompensating for thermal-related errors in accordance with the presentinvention can be used with prevailing torque-locking fasteners toimprove the accuracy of ultrasonic load measurement during assembly.

It will be understood that various changes in the details, materials andarrangement of parts which have been herein described and illustrated inorder to explain the nature of this invention may be made by thoseskilled in the art within the principle and scope of the invention asexpressed in the following claims.

1. A method of making a load indicating, thread-forming fastener,comprising the steps of: providing a fastener having a first endincluding a surface for receiving an ultrasonic transducer, for makingultrasonic load measurements in the fastener, a shank extending from thefirst end and including thread-forming portions for tapping a hole, anda second end, opposite the first end and including a surface forreflecting an ultrasonic wave back to the first end; and attaching theultrasonic transducer to a head associated with the first end of thethread-forming fastener, for engagement by a tool for applying a torqueto the fastener.
 2. The method of claim 1 which further includes thestep of permanently attaching the ultrasonic transducer to the fastener.3. The method of claim 2 which further includes the step of permanentlyattaching an ultrasonic transducer comprised of a piezoelectric polymerfilm to the first end of the fastener.
 4. .The method of claim 2 whichfurther includes the step of vapor depositing an ultrasonic transducercomprised of an oriented piezoelectric thin film directly onto the firstend of the fastener.
 5. The method of claim 1 which further includes thestep of temporarily attaching the ultrasonic transducer to the fastener.6. A method of making a load indicating, thread-forming fastener,comprising the steps of: providing a fastener having a first endincluding a surface for receiving an ultrasonic transducer, for makingultrasonic load measurements in the fastener, a shank extending from thefirst end and including thread-forming portions for tapping a hole, anda second end, opposite the first end and including a surface forreflecting an ultrasonic wave back to the first end; attaching theultrasonic transducer to a head associated with the first end of thethread-forming fastener, for engagement by a tool for applying a torqueto the fastener; and applying an identifying element to the fastener,wherein the identifying element includes data associated with thefastener.
 7. The method of claim 6 which further includes the step ofapplying a permanent mark to the fastener.
 8. The method of claim 7which further includes the steps of applying a bar code directly to thefastener, and applying the ultrasonic transducer over the bar code. 9.The method of claim 6 which further includes the step of applying theidentifying element to an end surface of the fastener.
 10. The method ofclaim 6 which further includes the step of applying a non-volatilememory device to the ultrasonic transducer.
 11. The method of claim 10which further includes the steps of providing an exposed conductivelayer on portions of a top surface of the non-volatile memory device,and electrically connecting the exposed conductive layer to a conductivelayer on a bottom surface of the non-volatile memory device, placing theconductive layer on the bottom surface in close proximity to an activeelement of the ultrasonic transducer so that the bottom conductive layerfunctions as an electrode.
 12. The method of claim 11 which furtherincludes the step of electrically connecting the conductive layer on thetop surface to the non-volatile memory device, for communicating withthe memory device.
 13. The method of claim 10 which further includes thestep of applying a radio frequency identification device to theultrasonic transducer.